1. Field of the Invention
The present invention is directed to a method for calculating the space-time temperature distribution in and on an electron-irradiated anode of an X-ray tube for determining the load of the X-ray tube in a load computer for calculating use in the temperature distribution of an anode of an X-ray tube and is also directed to an X-ray apparatus having such a load computer for performing such a method.
2. Description of the Prior Art
As known, is the generation of X-rays ensues by irradiating an anode with electrons proceeding from a cathode. There is the known problem that only one percent of the electron beam energy is converted into the desired X-radiation, even given an anode surface of tungsten (high atomic number Z, Z=74). A large part of the electron beam energy merely heats the anode material. In the case of an X-ray tube, the remaining beam energy is scattered back into the inside of the housing of the X-ray radiator. The electron irradiation of the anode must therefore be interrupted when temperatures that reach or, respective, exceed the respective, maximally permitted operating temperature are reached in the anode block composed of various materials. On the other hand, the system is not optimally utilized if a premature shutoff of the X-ray apparatus occurs.
This problem is in fact usually alleviatedxe2x80x94but not eliminatedxe2x80x94by anodes rotating at high speed.
The temperature distribution of the anode must thus be acquired for protecting the X-ray tube. The thermic condition of the anode can thereby be mensurationally or computationally acquired. Since the thermic condition of the anode, particularly the condition at individual anode locations, is extremely difficult or, impossible (at inside anode locations) to identify by means of measurements computational determination methods are utilized. In the computational acquisition of the thermal condition of the anode, a computer permanently determines the respective temperature distribution of the anode, for example from the cumulative loads and the cooling curve, and indicates them, for example, as percentage heat unit (HU) values. The waiting time after an X-ray exposure can be determined from the selected data for the following load and displayed with the assistance of fast micro-computers. Such a computer, called a tube load computer, or load computer can therefore optically and/or acoustically indicate inadmissible conditions for the X-ray apparatus to the operator and/or control the X-ray apparatus according to the calculated temperature distribution.
Load computers hitherto employed are based on simpler physical models. This can lead thereto that the X-ray means is in part prematurely shut off and, thus, an optimum utilization of the X-ray means is prevented.
Further, theoretical calculations of anode temperature distributions are known. Simple one-dimensional and two-dimensional model calculations about the anode surface temperature are known, for example, from G. E. Vibrans, xe2x80x9cCalculation of the Surface Temperature of a Solid under Electron Bombardmentxe2x80x9d, MIT Lincoln Laboratory, Technical Report No. 268, 1962, or from S. Whitaker, xe2x80x9cX-Ray Anode Surface Temperatures: The Effect of Volume Heating, SPIE Vol. 914, Medical Imaging II, 565, 1988. More involved calculations of the anode temperatures are known, for example, from H. Dietz, E. Geldner, xe2x80x9cTemperature Distribution in X-Ray Rotating Anodesxe2x80x9d, Part 1. Physical Principles, Siemens F and E-Ber., 7, 18, 1978. These known techniques cannot assure that the X-ray tube is optimally utilized due to an exact calculation of the temperature distribution of the anode.
An object of the present invention is to enable an improved operational use of X-ray systems in that the temperature development and distribution of the anode is computationally determined better than before.
The point of departure of the invention is to determine the space-time temperature distribution in the anode from two different contributions, namely from the short-term temperature boost in and around the focal spot during and immediately after the brief-duration electron bombardment of the focal spot, as well as from the long-term space-time temperature distribution in the overall anode volume as a result of the heat propagation that proceeds from the focal spot and as a result of the heat emission from the anode surface. Accordingly, the mathematical-physical model of the anode is composed of two independent sub-models, namely a short-term load model and a long-term load model.
xe2x80x9cShort-termxe2x80x9d in the sense of the present specification thereby indicates a time span wherein the electron bombardment of a focal spot ensues. This is usually a time span in the range from approximately 10 through 100 xcexcs.
xe2x80x9cLong-termxe2x80x9d, in contrast, indicates a time span wherein the overall image data of an X-ray exposure are usually acquired, i.e. usually more than approximately 1 s.
According to the invention, a method is provided for calculating the space-time temperature distribution in an electron-bombarded anode of an X-ray tube. The short-term temperature boost is thereby measured in a surface layer in and around a focal spot on the anode for the time span during and immediately after the electron bombardment of the focal spot, being calculated according to the general thermal conduction equation for uniform heat conductors. Further, the long-term temperature distribution in the overall volume of the anode is calculated taking the heat propagation that proceeds from the focal spot and the heat emission from the surface of the anode into consideration, being calculated according to the general thermal conduction equation for non-uniform heat conductors. The results of the two calculations are then added for determining the temperature distribution on or, respectively, in the anode. The result of the calculation, the load of the X-ray tube, can be displayed for the user and/or taken into consideration in the drive of the X-ray tube. These calculations of the temperatures of the anode make it possible to protect the X-ray anode against destruction due to overheating. The X-ray generator can be deactivated shortly before the upward transgression of permitted maximum temperatures at selected anode locations such as, for example, in the focal ring or in the boundary layer between anode material. Further, the method can be employed to calculate in advance whether an X-ray examination can still be carried out in view of the thermal load on the anode or whether a pause for cooling the anode is required.
One or more of the following factors can be inventively taken into consideration in the calculation of the short-term temperature boost:
The backscatter of the bombarding electrons in the form of a multiplicative factor  less than 1. This factor thus reproduces the reduction in the power supplied to the anode due to the backscatter.
Given movement of the anode during the bombardment, the relative motion of the electron beam with respect to the anode can be taken into consideration in the calculation of the short-term temperature boost by topical variation of a heat source function.
Given non-homogeneous profile of the electron beam, the inhomogeneity of the beam profile can be taken into consideration in the calculation of the short-term temperature boost by discretizing the area of the focal spot into individual area elements.
At least one of the following factors can be inventively taken into consideration in the calculation of the long-term temperature distribution:
the backscatter of the bombarding electrons in the form of a multiplicative factor that is less than 1, whereby this factor can be different (usually greater) than the backscatter factor in the calculation of the short-term temperature boost.
The three-dimensional heat flow by describing the volume of the anode as cylinder, whereby the cylinder is composed of one material layer or is a composite of a plurality of layers of different materials.
The radiation exchange between the surface of the anode and the environment (housing) of the anode, as well as
the temperature dependency of the material parameters.
Inventively, a load computer for calculating the temperature distribution of an anode of an X-ray tube is also provided. This load computer is programmed to execute the aforementioned method and has a display for displaying the results of the calculations, and controls the operation of the X-ray tube dependent of the results of the calculations.
According to the invention, further, an X-ray apparatus is provided that has a load computer as described above. The anode in the X-ray apparatus can be a rotating anode.
Further, the surface layer of the anode of the X-ray apparatus can contain tungsten, a further layer in depth the direction can contain molybdenum, and yet another layer can contain carbon.